Default Printing Speed: This is the speed at which the Infill of your part will be printed, assuming the Cooling Overrides don't adjust it and that it's not a 100% infill layer.
Outline Underspeed: For your perimeters, you print at a % of your Default Printing Speed, this is the %. For instance, if you want to print your Perimeters at 75% of your Default Print Speed.
Solid Infill Underspeed: For your any layer that is 100% infill, including Bottom Solid Layers, Top Solid Layers and if you have 100% infill set in your part, you print at a % of your Default Printing Speed.
Support Structure Underspeed: The support material will be printed at a percentage of your default printing speed.
X/Y Axis Movement Speed: This is the speed your printer will move when your printer is not printing in the X-Y axis. When looking at the G-Code previewer, you can see these lines as the Travel Moves
Z Axis Movement Speed: The speed at which your Z-axis will move.
Filament Diameter: The diameter of filament your printer uses. This is very important for extrusion values. For instance, if you use 3 mm filament but have 1.75 mm filament in your setting, the software will be turning your extruder motor FAR too much since it will think you are using only 1.75 mm diameter filament. Small tweaks to this would be the same as small tweaks to your extrusion multiplier.
Filament Price: Price that you paid for your filament per kg, so that when you slice your file in the G-Code preview you can see the estimated cost.
Unsupported area threshold: Any area larger than this that is unsupported will have the bridging multipliers applied to the infill in the bridging.
Bridging extrusion multiplier: If you find that extruding a lot more/less material helps with bridging, then you would set that here.
Bridging Speed multiplier:If you find that printing faster/slower helps with bridging, then you would set that here. One trick I use occasionally, is I'll temporarily set the bridging speed multiplier to 999, then slice my file, that way the areas in which the bridging multiplier is taking place are bright red and stand out in the G-Code previewer, that way I have a better idea of where the bridging settings are occurring.